Method of processing steel



United States Patent 3,502,514 METHOD OF PROCESSING STEEL Raymond A.Grange, Washington Township, Westmoreland, Pa., assignor to UnitedStates Steel Corporation, a corporation of Delaware No Drawing. FiledJan. 30, 1968, Ser. No. 701,554 Int. Cl. C21d 7/02, 1/00 US. Cl.148-12.4 8 Claims ABSTRACT OF THE DISCLOSURE A method of processingcarbon and low-alloy steels to develop a fibrous martensite-ferritemixture including the steps of cold reducing steel which is preferablyin the hot-rolled or heat-treated condition to at least a 50% smallercross sectional area than initially to lengthen same, and then heatingto a critical temperature (T which is determined for the steel by aspecified equation and thereafter quenching to transform austenitefibers to martensite.

This invention relates to a process for strengthening hypoeutectoidsteels. More particularly, the invention involves a method of processinghypoeutectoid steels to develop a fibrous martensite-ferrite mixture incarbon and low-alloy steels.

Processing condition for producing microstructures comprised ofmartensitic particles imbedded in a matrix of ferrite are usuallyavoided because such metallurgical structures have been though of ashaving inferior mechanical properties. As a result, with very fewexceptions carbon and low-alloy steel products are produced by processeswhich develop microstructures comprising carbide particles in ferrite.Recently, it has been determined that if martensite particles aresufiiciently small and all elongated in the same direction, theresulting fibrous martensite-ferrite mixture may have useful properties.In fact, it has been found that such a metallurgical structure has aunique and useful combination of mechanical properties and the presentinvention is directed to a particularly effective method of processingsteel to develop a fibrous martensite-ferrite mixture in a variety ofcarbon and low-alloy steels.

The process in accordance with the invention involves treating asemi-finished steel of larger cross section than the product and theprocess itself includes the steps of cold reducing the cross section atleast 50% to desired dimensions by some cold deformation process,reheating to a particular closely controlled temperature and quenching.Initially the steel may be in a hot-rolled condition or it may have beenheat treated to enhance ductility. Cold deformation may be accomplishedby any means which reduces the cross section (i.e. cross sectional area)and hence lengthens the material. Examples of such cold deformationtechniques include rolling, drawing, extrusion and swaging. In theseocnd step which may follow the cold deformation in an uninterruptedsequence or may be performed later as a separate treatment, the steel isreheated to a controlled temperature within the critical range of thesteel. This temperature (T is very important and varies with thechemical composition of the steel being processed. However, T can becalculated from the chemical composition of the steel by the followingequation:

Other elements may be present in the steel but are not considered in theequation because they do not significantly affect the reheat temperaturein the steels with which the invention is concerned.

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In accordance with the invention, steel which has been cold worked asdescribed above is desirably heated to as close to the T temperature aspossible and a reheat temperature within :20 F. of T will usually besatisfactory. It is not necessary to hold at the T longer than requiredfor the steel to reach T 2O F. throughout Although considerable leewayin heating time is permissible, prolonged times beyond 10 minutes areundesirable. Any method of heating may be used which permits the degreeof the maximum reheat temperature throughout the steel within thespecified T -20 F. range. Preferred methods of heating includeelectrical induction or resistance heating or immersion in lead or saltbaths. After reheating, the steel is quenched (preferably with water) toroom temperature.

A consideration of the microstructural changes in the steel indicatesthe reason for processing the steel as described above in accordancewith the invention. The normal starting material for the process willordinarily be hot rolled and have a microstructure of equiaxed ferriteand pearlite. The cold reduction step deforms pearlite colonies andferrite grains in a manner corresponding to the change in the dimensionsof the material as a whole. Thus, both pearlite colonies and ferritegrains are elongated in the direction of principal deformation. Thegreater the amount of cold reduction, the longer and thinner thepearlite colonies and ferrite grains become. To achieve the desiredelongation of the pearlite colonies, cold reduction of greater than 50%is required. This amount of reduction may, of course, be produced in anyconvenient manner, usually for example, by a number of deformation stepssuch as a plurality of passes through a rolling mill. The process inaccordance with the invention does not require a segregated or so-calledbanded condition in the steel initially.

On reheating to the T temperature which is somewhat above the Ac theelongated pearlite converts to austenite. Austenite forms almostexclusively in the pearlite and hence assumes a corresponding size,shape and distribution. The result is that just prior to quenching, thesteel contains austenite particles which are long and thin, and alloriented alike, and having substantially the same chemical compositionas the pearlite. Control of the reheat temperature T is necessary toinsure that austenite particles of this nature are present. If thereheat temperature is too low, pearlite is not fully converted toaustenite. If too high, austenite grows into the ferrite andconsequently is reduced in carbon content and becomes less fibrous.Also, in reheating, the cold worked ferrite matrix recrystallizes intoannealed equiaxed grains. Because the reheat temperature is relativelylow and the heating time short, a desirable fine grain size is developedin the ferrite.

Quenching to room temperature transforms the austenite fibers intomartensite fibers. The hardenability of the austenite fibers is muchgreater than that of the fully austenitized steel because the fibershave substantially the composition of pearlite and hence contain closeto the eutectoid carbon cont nt. Moreover, because of the aifin ity ofcarbon for manganese, this element is also present to a greater extentin the fibers than in the steel as a whole. These features make itpossible to apply the process to steels which contain substantiallylower carbon and alloy contents than is otherwise required for quenchingto all martensite structures.

The product produced by the process described herein contains amicrostructure which comprises martensite fibers containing up to about0.8% carbon imbedded in a fine-grained ferrite matrix. The fibers are,therefore, about 10 times as strong as the matrix. This microstructure,therefore, is a desirable composite material since the strong martensitefibers tend'to strengthen the more duetile ferrite matrix. Carefulcontrol of the reheat temperature, pointed out above, is essential toachieve this condition.

Subsequent processing may be performed to modify The strengths obtainedin the various representative hypoeutectoid carbon and low-alloy steelsin Table II are the result of processing to develop a fibrousmartensiteferrite microstructure. Tempering at 400 F. as the samples inTable II were, further improves the ductility properties. Ductility isincreased at some sacrifice in 5 and yield strength combination. If thetempering step is strength by tempering, e.g., at about 400 F. Higheromitted, substantially higher tensile strengths may be obstrength andbetter machinability may be obtained by cold tained but with lowerductility. As indicated previously, working as necessary, e.g. to up toabout a 20% reduction additional strength and enhancement ofmachinability is in cross section. imparted by cold working asnecessary, e.g. up to about It is within the purview of the invention totreat hypoa 20% reduction in cross section. eutectoid steels and inparticular those preferably con- The fibrous martensite-ferrite mixtureis a type of taining less than 0.35% carbon. However, carbon as wellmicrostructure which is not developed by conventional as low-alloysteels can be processed by the invention, p ng r h at treat ng, and theproperties resulting although mechanical properties after processingwill, of therefrom include potentially useful characteristics such ascourse, vary with the type of steel. The following is unusually highrate of work hardening, relatively good provided as further examples ofthe process as described machinability and resistance to certain kindsof corrosive herein. attack.

A 1-inch diameter hot-rolled bar of A181 1015 steel In view of theforegoing, it is apparent that various containing 0.16% carbon and 0.51%manganese was cold chan and modifications m y m d Wit ut departswaged to0.25-inch diameter. It was then reheated by g from the ihVehtiOIl-Aeeerdingly, the Scope of the immersing in a lead bath at 1380 F. forZ-minutes and invention should be limited only by the appended claimsthen water quenched. This specimen is designated as Exwhereby What isClaimed ample A in Table I below. For comparison, an identical 1. Amethod of processing carbon and low-alloy steels 0 25-i h di t d was lid f 1700 F, to developafibrous martensite-ferrite mixture which comnd isinclud d in T bl I a Sample B, T illu tr t furprises cold reducing anarticle of said steel to at least a {her improvements b i bl a specimeni il t 50% smaller cross sectional area than initially to elongateSample A was additionally tempered at 400 F. for 1- said article,heating said cold reduced article to a temperhour and is reported inTable I as Sample C to show attire in the g x) Where TX is deterhow adifferent combination of yield strength and ductilmilled y the followingequation: ity may be obtained within the purview of the invention. F)=1385 (25X%M11),+(40X%Si) TABLE I (26 %Ni)|(42 %Cr) Tensile Yi 81d andquenching to transform austenite fibers to martensite. strength strengthRed. of area Elo11g. in 1- 35 2. A method in accordance with claim 1wherein the (percent) (percent) starting material is in the hot-rolledcondition. 136,900 09,100 19 12 3. A method in accordance with claim 1wherein the 33333 1238 starting material is in the heat-treatedcondition.

4. A method in accordance with claim 1 wherein the product producedthereby is cold worked to enhance The examples described above in TableI demonstrate the Strength and improved machinability Strength thatresults in a low-carbon Steel y Processing 5. A method in accordancewith claim 4 wherein said in accordance with the inventionproduct iscold worked up to about 20%.

Tensile Properties of Strip of Several grades of Steel 6. A method inaccordance with claim 1 wherein, after Pmeeseed according to the Processhereof and all quenching, said article is tempered to improve ductility.sequehtly tempered at F for z'hotlrs are desel'lbed 7. A method inaccordance with claim 6 wherein said in Table II. With each steel of theexamples the starting tempering is at material WaS- 0.5-inch thhot-felled p The Plates 8. A method in accordance with claim 6 wherein,after were cold T l d t0 5- reheated t0 Txizoo and tempering, the steelis cold worked to enhance strength water quenched. and improvemachinability.

TABLE II Tensile Yield Elong. in

AISI Grade 0 Mn s1 Ni Cr M0 531%? 831%? ereiiiiii 1010 0. 11 84, 100 55,800 22. 5 1011 (Mod.) .12 118,700 79,600 19 1020 20 91, 700 68,700 21 24137,10 78, 200 13. 5

References Cited UNITED STATES PATENTS 3,423,252 1/1969 Grange 148-12 L.DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner

